198 SUPPLEMENT TO THE 



description of JS. deltoiclea, there are descriptions of four other shells there given under 

 Sfrojj/iomena, all of which are mentioned as having the inferior valve convex, but no 

 cardinal area is mentioned. 



" Conrad undoubtedly included both the above under his S. deltoidea. The figure 

 given by Vanuxem, ' Geol. Rep./ Third District, New York Survey, p. 46, fig. 2, would 

 do for either except for the little beak projecting beyond the hinge. Hall has figured 

 shells of S. deltoidea, but none of 8. incrassata, vol. i, also S. alternata, fig. 3 a, h, and c, 

 and S. tenuisfriata, fig. 3 a, as deltoidea, so he has not cleared the matter away. Now, the 

 one which 1 identify as S. incrassata, SafiFord (not Hall) is the same, I believe, as your 

 S. deltoidea, Sil. Mon., PI. XLH, figs, 1 — 5, and not the S. deltoidea proper." 



There evidently prevails among American geologists some uncertainty with respect to 

 Conrad's Stroph. deltoidea. Dr. C. Callaway, who has visited and collected at Trenton 

 Falls, sends me, as Conrad's species of S. deltoidea, two specimens, which are exactly 

 similar to the British examples I have figured and described in my ' Monograph.' 

 They agree likewise with a figure of S. deltoidea given by Hall in PI. 31a, fig. 3/, in 

 vol. 1 of his ' Palaeontology of New York ; ' Hall describes his figure as follows : " — A very 

 perfect specimen in form and markings. This drawing is by Mr. Conrad from a specimen 

 now in my possession. The striae are a little stronger on the middle of the shell, and in 

 some parts alternate with smaller ones." Unfortunately, all Hall's figures of S. deltoidea 

 represent only the convex valves, and no area is given, but he tell us in his description 

 that the dorsal valve (our ventral one) is convex. 



Now, the specimens collected by Dr. C. Callaway at Trenton Falls agree with Conrad's 

 drawing of 8. deltoidea figured by Hall, and have the ventral valve convex with area, 

 the dorsal valve being the concave one, and the same striations as described by Conrad ; 

 consequently I am led to infer that Dr. Callaway's two examples do really belong to 

 Conrad's S. deltoidea ; and, as they agree with our British representative of the species, we 

 will be justified, I think^ in retaining them as such. The impressions taken from the 

 American specimens of S. incrassata would not agree with ours. Dr. Callaway found 

 another species of Strophomena\ in the Hudson-River group, Cincinnati, Ohio, and in 

 this the ventral valve with area is the concave one ; but it is evidently not the S. 

 deltoidea of Conrad, although he sent me the specimens for examination with that name. 



Strophomena deltoidea is stated by Hall to be very variable, and he says that it is often 

 very difficult to draw the line of distinction between this species and L. alternata, and 

 more particularly between this and L. camerata ; and that, on the other hand, it 

 approaches very close, in some of its forms, to the succeeding species (Z. tenuistriata) ; 

 but that these two, when well preserved, are clearly and decidedly distinct; also that the 

 figures of S. deltoidea given by Mr. Vanuxem and Dr. Emmons scarcely show any 

 concentric wrinkles, and approach more nearly to the nasute form of Z. alternata. 



British examples of S. deltoidea are also very variable in shape, the ventral valve 

 being more or less strongly geniculated, while some, as in the case of the American 



